1. Field of the Invention
The field of the invention is the writing of information to a recordable optical storage media in an optical disc drive, more particularly, the implementation of near-optimal recording power when writing to a multi-session recordable optical disc.
2. Description of the Prior Art
Optical discs are a versatile and cost-effective means of storing digital data and have been considered standard equipment on personal computers for several years. Broadly speaking, optical disc drives can be separated into two categories: read-only devices, which can only be used to read information already stored on an optical disc, and read-write devices. Devices in the latter category, as well as having the ability to read stored data from an optical disc, have the additional capability to record data. Some examples of read-only (read-only memory or ROM) optical disc drives are compact disc drives (CD-ROM), and digital versatile disc drives (DVD-ROM) in the prior art. An example of a device capable of writing to optical discs is a DVD+RW drive, this being a type of digital versatile disc—read-write/digital versatile disc—re-writable drive.
Optical discs store data as a continuous track of ‘pits’ (or ‘marks’) and ‘lands’ (or ‘space’) on a data-bearing surface that is rendered reflective by the application of a metallic layer during manufacture, the aforementioned ‘lands’ simply being parts of the track that are not pits. The continuous data track is formed on the optical disc in an Archimedean spiral, the start of the data area generally being toward the center of the disc and so in most cases data is read back from inner radii to outer radii. Pits/marks in ROM discs are molded into the data bearing surface when the discs are formed, whereas recordable and re-writable discs are produced as blanks, and have only a preformed groove or ‘pre-groove’ (together with a limited amount of embossed data in most cases) included during molding. Data is stored on re-writable optical discs using the same pit-land/mark-space principle, however the pit/mark features are added by ‘burning’ a special phase-change material layer applied to the disc substrate. In order to ‘burn’ or write to a re-writable disc, an optical pick-up head of an optical disc drive must be equipped with a ‘write laser’ in addition to a ‘read laser’, although these two items are generally a single laser being capable of operating at lower power output for read operations, and a range of higher power outputs for write operations.
The amount of power used to ‘burn’ pit/mark features into the bottom of the pre-groove, is crucial to the geometry of said features. The geometry of the pit/mark features in turn effects the read-back performance of read-back systems, hence prior to writing to a re-writable optical disc, a prior art optical disc drive will perform a write laser power calibration sweep known as optimum power calibration (OPC). Please refer to FIG. 1, which shows the general layout of a prior art single session recordable optical disc 10, featuring a lead-in area 11, an embossed area 12, a data area 13 and a lead-out area 14. The OPC generally comprises writing a test pattern to a designated portion of an optical disc positioned before the start of lead-in (the beginning of the user data area) referred to as the power calibration area (PCA). Alternatively, a PCA may be located after the end of lead-out (the end of the user data area).
Because there are two basic techniques applied to controlling the rotational speed of discs in optical disc drives, the recording power calibration value, obtained by the OPC process carried out in the PCA, may not be valid across the whole of the recording area. The two techniques used to control the rotational speed or disc velocity mode of optical discs are constant linear velocity (CLV) mode, and constant angular velocity (CAV) mode.
CLV mode is generally used for recording audio and video data, the speed of the pre-groove relative to the optical pick-up head (and therefore the read/write laser spot) being kept constant throughout the data area. This means that the disc is required to spin faster when the pick-up head radial position corresponds to the inner radii of the disc, i.e. toward the center of the disc, and slower when the pick-up head radial position corresponds to the outer radii of the disc. But because the pre-groove/laser spot speed relationship remains constant, the amount of laser power required to alter the reflectivity of the phase-change layer and so form marks in the pre-groove, also remains constant. That is, the amount of laser power required to produce a readable mark at radius 25 (25 mm from the absolute center of the disc) is the same as that required to produce a readable mark at radius 60. Were it not for other factors that effect recording power in CLV mode (some of which will be discussed later), an OPC implemented in the PCA should produce a recording power setting that can be applied at any point on the disc. In contrast, CAV mode applies a constant angular velocity to the disc, in other words the disc is spun at a fixed speed regardless of pick-up head position, and is more commonly used in computer data applications where rapid access times are an important factor.
A flow diagram outlining the prior art DVD+RW process including OPC, is shown in FIG. 2. The flow diagram 20 of FIG. 2 includes the following steps:
Step 1000: Start.
Step 1001: A decision is made regarding the write strategy to be used according to the considerations mentioned above, i.e. CLV is generally selected for concatenated data (hence the process will proceed to step 1002), audio and video for example, and CAV where random access is required within the session and perhaps the whole disc (hence the process will proceed to step 1004), as is generally the case for computer data applications.
Step 1002: In the case of CLV mode being selected, the optical disc drive pick-up head moves to the inner PCA located prior to (radius-wise) the lead-in area.
Step 1003: A Standard OPC is carried out at the inner PCA and the process proceeds to step 1009.
Step 1004: In the case of CAV mode being selected, the optical disc drive pick-up head moves to the inner PCA located prior to (radius-wise) the lead-in area.
Step 1005: A Standard OPC is carried out at the inner PCA.
Step 1006: The optical disc drive pick-up head moves to the outer PCA located after (radius-wise) the lead-in area.
Step 1007: A Standard OPC is carried out at the outer PCA.
Step 1008: Because a ramped recording power profile is required for recording, an interpolation technique is used to derive a suitable profile according to the OPC outputs, the selected recoding speed and (optionally) disc information such as phase-change material type.
Step 1009: The optical disc drive pick-up head moves to the lead-in start address of the new session.
Step 1010: The optical disc drive pick-up head write laser writes the session data to the disc, including (session) lead-in and (session) lead-out.
Step 1011: The session is closed.
Step 1012: End.
Because in CAV mode the disc is always rotating at the same speed, data stored at various different radii can be retrieved without long delays caused by the need to adjust and settle out the read-back speed. This in turn though, means that in CAV mode the relative velocity of the pre-groove and the read/write laser spot increases with radius, and with it the amount of laser power required to make a readable mark in the phase-change layer also increases. Hence, in CAV mode, an OPC implemented in a PCA, whether that PCA is located at the inner or outer radii of the recordable data area, will not be valid for all points on the disc. Hence commonly in the prior art, a power ramp profile is applied to the OPC derived recording power value to compensate for the abovementioned increase in relative velocity.
However, a simple linear ramp profile fitted to the increase in relative velocity is not adequate, because a number of factors introduce non-linearities to the speed/power relationship. For example, the type of phase-change material used and its application can vary from disc to disc and from manufacturer to manufacturer. Many discs do not have a linear response to increases in power, i.e. a doubling in the relative velocity between the pre-groove and the read/write laser spot may require more than double the amount of recording power. So, while a more accurate recording power ramp profile can be modeled using empirical data, the prior art method of carrying out OPC only in the PCA portion of a disc cannot optimize recording power for a particular disc. (Even when a power ramp is not required, as in CLV mode, the issue of disc manufacturing process variations local to the instant recording point will affect optimal recording power).
In addition, the abovementioned disc manufacturing process variations are typically introduced in circumstances where although process control may perhaps be within limits, it is not optimal. Because such variations are departures from the disc specification, it's unlikely that they will feature in the information encoded by the manufacturer in the disc pre-groove, hence end-user optical disc drives will receive no warning of their existence.
The problems relating to the recording of single session discs so far discussed, may also be encountered when recording a multi-session disc. When using, for example, an optical drive to record archive data onto a recordable disc, the entire capacity of the disc is not always filled during one recording session. Therefore, to maximize the utilization of recordable discs, multi-session strategies exist in the prior art to allow additional sessions to be appended after a first session, providing there is adequate disc space to accommodate the session(s) to be appended. Each appended session comprises a similar structure to that found in a single session, having autonomous lead-in, data and lead-out areas. Please refer to FIG. 3, which shows a general layout of a prior art multi-session recordable optical disc 30 featuring a first session, session 1 (31), comprising a lead-in area 32, a data area 33 and a lead-out area 34. A second session is appended, session 2 (35), comprising its own lead-in area 36, data area 37 and lead-out area 38.
It can be appreciated that the prior art methods discussed, being dependent upon data obtained by performing optimum power calibrations in power calibration areas located at the extremities of a recordable disc, carry drawbacks with respect to anticipating disc conditions in areas other than PCAs. Therefore they are unlikely to be able to maintain truly optimal recording power settings across the disc, whether writing single or multi-session recordable discs. There is a requirement then, to look beyond the prior art methods, for a method that can derive a recording power for an optical disc drive write laser, which is more closely optimized to the point of recording.